A variety of terms and concepts have been used to describe aspects of the perception of body movements and positions, for example

kinesthesia,

proprioception,

somaesthesia,

haptic system or touch,

position sense or movement sense

muscle sense or joint sense, or

sixth sense.

The internal / external distinction in concepts of proprioception / exteroception are not valid criteria for classifying sensory systems and so a concept of kinesthesia as arising in both internal & external data from receptors throughout the body offers an integrated model.

Variety of terms and concepts

A variety of terms, including kinesthesia, proprioception, somaesthesia, the haptic system, position sense, muscle sense, joint sense, and movement sense, have all been used in similar ways to describe aspects of the perception of bodily movements and positions.

Sixth sense.

The sense of bodily movement and position of ones own body does not fit easily into Aristotles classic senses; seeing, hearing, smelling, tasting, and touching. Consequently Sir Charles Bell (1833) and others more recently (Fitt, 1988, p. 266) use the notion of a sixth sense.

Dickinson (1974, p. 9) explains how the five senses were based on a doctrine of specific nerve energies whereby a particular sense is thought to emerge from a particular sensory receptor. This has been shown to generally not hold true. For example, audition and vision both utilise data from head and body movements which are used to achieve a variety of postural orientations from which to sample the visual or audio stimuli (Scharf and Houtsma 1986; Sedgwick 1986). This is especially true for the perception of bodily movements and positions which arises from receptors throughout the body, including muscles, joints, tendons, skin, labyrinth, visual, audio, and an efferent discharge loop (see below). Kinesthesia is not a new "sixth sense" but refers to perceptions which arise from various receptors located throughout the body:

. . . the doctrine of specific nerve energies [which was] held earlier this century has blinded later researchers to the fact that movement sensitivity does not depend on specialised receptors. It is not simply a sixth sense to be added to Aristotles five classical senses.

(Dickinson, 1974, p. 9)

Touch.

The sense of bodily movement and position can also be conceived to be an expanded generalised version of Aristotles sense of touch. Bastian (1888, p. 5), who proposed the term kinesthesia (see below) noted that its cerebral seat or area corresponds with the sense of touch. Rock and Harris (1967, p. 96) also use touch in this broad definition according to which Touch includes several other components [in addition to skin sensitivity], of which the one most significant for this discussion is the position sense.

Indeed, the sense of touch, or cutaneous sense, is itself not entirely based on data from skin receptors but also relies on sensations from muscles, tendons, and joints to perceive the shapes of objects being touched by moving the body around the object, and to perceive the texture of an object by exerting variations in pressure (Gibson, 1966, pp. 50, 53; Schwartz et al., 1975).

The sensations of bodily movements, positions, and touch, all arise from the same system of receptors and so they can be conceived to belong to the same sensory system. The specialised touch sub-system can be included together with the other specialised kinesthetic sub-systems including the sense of balance, force, linear or rotary self-motion, sense of limb movement and limb position (see below).

Kinesthesia.

Bastian (1888) proposed the term kinesthesis to refer to the sensations which result from or are directly occasioned by movements (p. 5). Despite its etymology (Greek kinein, to move + aisthesis, feeling; American, 1982]) meaning literally the feeling of movement, this term was intended to replace both the terms muscular sense, and sense of force which were previously in use (p. 5). Thus, under the heading of kinesthesis he included the perception of position and movements of our limbs, and different degrees of resistance and weight (p. 6). The dictionary definition of kinesthesia also groups together the various sub-senses within its class, including bodily position, weight, muscle tension, and movement (Collins, 1986) and also the sense of presence (American, 1982).

The term muscle sense may be synonymous with kinesthesia (Collins, 1986) in that kinesthesia was adopted to replace the former term (Clark and Horch, 1986). Alternatively, the muscle sense may be considered to be one of the several kinesthetic sub-senses, including muscle sense (receptors in muscles), tendon sense (receptors in tendons), joint sense (receptors in skeletal joints), and static sense (receptors in the labyrinth) (English and English, 1974).

The common accepted usage is that kinesthesia refers to the perception of one's own bodily movements and positions (Clark and Horch, 1986, Fitt, 1988, p. 266), and sometimes also the forces produced or reacted to (Rasch and Burke, 1978, p. 80). Some authors also include processes of motor coordination and motor memory as part of kinesthesia (Fitt, 1988, p. 267).

In contrast, Cross and McCloskey (1973, p. 443) distinguish between position sense for limb positions and kinesthetic sensations for limb movement. McCloskey (1973) vibrated the tendon of subjects biceps brachii muscle with a physiotherapy vibrator which induced the subject to perceive an illusion of movement at the elbow joint. The illusions of movement and illusions of position were able to be experimentally manipulated so that they did not correspond: A longer duration movement illusion did not result in different position illusions; A low frequency vibration did not cause a movement illusion but did create an illusion of a changed position:

. . . subjective judgements of the static positions of joints and judgements of movements of joints can use different lines of information. It is suggested that the term position sense be reserved for the static judgements, and kinesthesia for the dynamic ones, and that the two terms should not be regarded as synonymous

. (McCloskey, 1973, p. 130)

Proprioception.

Sherrington (1906) uses the term proprioception in his discussion of how types of sensory impulses will initiate certain physical reflexes. Sherrington draws a general distinction between internally versus externally produced sensory stimulation:

Multicellular animals . . . are cellular masses presenting to the environment a surface sheet of cells, and under that

[is] a cellular bulk [which is] more or less screened from the environment by the surface sheet. (Sherrington, 1906, p. 316)

Proprioceptors refer to the receptor cells which are screened from the exterior environment, and so the stimuli to the receptors are given by the organism itself (Sherrington, 1906, p. 130). The term derives from the Latin proprius (ones own) plus reception and so is defined as the reception of stimuli arising within the organism (American, 1982).

Sherrington (1906) distinguishes two types of sensory cells on the organisms exterior surface; exteroceptors and interoceptors. Exteroceptors refer to sensory cells which are freely open to the numberless vicissitudes and agencies of the environment (p. 317). Interoceptors refer to sensory cells on surfaces of the body, but which have developed deep recessions, in this recess a fraction of the environment is more or less surrounded by the organism (p. 317).

Vestibular labyrinth receptors form a special case. Sherrington (1906, p. 336) describes these as being derived from the extero-ceptive, but later recessed off from it and which now function as proprioceptors:

The proprio-ceptors of the body generally and of the labyrinth receptors in the head appear to co-operate together and form functionally one receptive system . . . embraced within the term proprio-ceptive.

Sherringtons distinctions are often followed closely (Dickinson, 1974, p. 10; Ellison, 1993, p. 75; Rock, 1968) and also have been misrepresented. For example Wells and Luttgens (1976, p. 58) consider proprioceptors (together with visceroceptors) to be a type of interoceptor, contrary to Sherrington who explicitly distinguishes between the two.

Somaesthesia.

Somaesthesia (or, somesthesis, somatosensory) is used very similar to proprioception in that they both refer to perceptions arising out of one's own body (from Greek soma, the body). Somaesthesia refers to sensations of body movements and positions, and also to sensations of temperature, pressure, touch, and pain (Collins, 1986). It is generally used to refer to stimulation arising from receptors in muscles, tendons, joints and skin (not vestibular) (Bles, 1981; Lackner and DiZio, 1984; Taub et al., 1973; 1975). The muscle/tendon/joint/skin conception of somaesthesia is sometimes considered to be synonymous with kinesthesis (English and English, 1974).

Haptic system.

Gibson (1966, pp. 50, 53) proposed a sub-group within proprioception which he termed the haptic system with its mode of attention as touching and using sensory data from receptors in skin, joints, muscles, and tendons to produce perceptions about the environment or the body. The hands and other body members are considered to be the organs of perception. The haptic system can derive information about ones own body or about the exterior environment (eg. feeling the shape of an object) and so is both proprioceptive and exteroceptive. Haptic is from the Greek haptein, to touch, (Collins, 1986) and so is identical with the generalised notion of the sense of touch (see above).

Discussion and Working Definitions

Sensation and perception of body movements and positions is described with a wide variety of terminology and concepts which organise the receptors and perceptions in different ways. Certain criteria can be identified to asses the usefulness of each concept.

Invalidity of Internal / External Distinction.

The proprioceptive/exteroceptive distinction between internal stimuli from the body versus external stimuli from the environment has been found to be invalid. In many cases external stimuli such as visual-field motion or audio-field motion (see below) provide information about the bodys movement. In his landmark work, Gibson (1966) explored how audio and visual perception are extremely important for the perception of ones own movements. It has been shown that the movement of audio or visual fields can easily induce illusions of self-motion even in the absence of joint, muscle, tendon and labyrinth stimulation (G. J. Anderson, 1986; see below). This type of external stimulation is also vital for maintenance of the bodys balance (Lee and Aronson, 1974; see below).

Receptors in skin which can receive stimulation from the exterior environment will also respond to stimulation from body movement. Thus these must be classified as both proprioceptors and exteroceptors. Bastian (1888), who proposed the term kinesthesia stated this same fact at the outset, that the group of sensations under the name of kinaesthesis . . . is confessedly a mixed group partly intrinsic and partly extrinsic in their origin (p. 6).

Inconsistent use of Kinesthesia vs Proprioception.

Another problem is that the terms kinesthesia and proprioception are not consistently defined. Typically the two terms are used synonymously (Clark and Horch, 1986; Schmidt, 1982, p. 202) and Moberg (1983, p. 1) considers kinesthetic sensibility, position sense, muscle sense or proprioception as synonyms. Similarly, sometimes the term visual proprioception(Lee and Aronson, 1974; Lee and Lishman, 1975), is used, while others refer to this as visual kinesthesis (Lishman and Lee, 1973; Rieger, 1983; Warren et al., 1988, p. 646).

The particular components included within kinesthesis or proprioception also vary among authors. In the narrowest view stimulation arising from receptors in muscles, tendons, and joints (not labyrinth or skin) are included as proprioceptors (Fitt, 1988, p. 266) or as kinesthetic (Laszlo and Bairstow, 1971).

In a slightly broader view, Bastian (1888, p. 5) considered receptors in muscles, tendons, joints and skin (but not vestibular) as being kinesthetic. Perhaps vestibular was not included because Bastian focused on the positions and movements of our limbs (p. 6) rather than linear or rotary self-motion (see below). Other writers also follow this same view of including stimulation from muscle, joint, tendon, and skin (but not vestibular) receptors as being kinesthetic (Clark and Horch, 1986) or as proprioceptive (Rothwell, 1987, p. 74), or as proprioceptive considered synonymous with somatic sensation (Taub and Berman, 1968). Similarly, Souder (1972, p. 14) considers the vestibular labyrinth to be a separate system from either kinesthetic or proprioceptive. Sherrick and Cholewaik (1986, p. 111-3) consider the cutaneous sense to be exteroceptive but also that the senses of the skin do occasional duty as supplement to the kinesthetic senses. Wells and Luttgens (1976, pp. 5861) include skin receptors as being proprioceptive only when they participate in withdraw and thrust reflexes.

In the broadest view, visual, audio, skin and labyrinth receptors are included together with receptors in muscles, tendons and joints as all contributing to kinesthesia (Rasch and Burke, 1978, pp. 80-81), or to proprioception (Gibson, 1966, pp. 36-37), or as kinesthesia considered synonymous with proprioception (Schmidt, 1982, chapter 6):

Historically, kinesthesis . . . was a term limited to a persons perception of his or her own motion, both of the limbs with respect to one another, and also of the body as a whole. Sherringtons (1906) term proprioception was originally used to mean the perception of movement of the body plus its orientation in space (even though it may not be moving). Over the years these two terms have become practically synonymous, and it is probably not important to continue this distinction.

(Schmidt, 1982, p. 202)

Other researchers might arrange kinesthesia and proprioception into a kind of hierarchy, however these arrangements tend to vary. The proprioceptive system is sometimes considered as a higher-order system containing the separate kinesthetic and vestibular systems (Riesser and Pick, 1976; Sherrick and Cholewaik, 1986). Strelow and Babyn (1981, p. 191) list vestibular, kinaesthetic, and proprioceptive information implying that the three are separate. Singleton (1972, p. 61) represents the somaesthetic system as containing the proprioceptive system and the tactile system. The proprioceptive system is then further subdivided as containing the kinesthetic system (including receptions from muscles, tendons, and joints) which is separate from the vestibular system.

Kinesthesia & Proprioception as Conscious & Unconscious.

Kinesthesia is sometimes used to refer to conscious perceptions since the Greek root aesthesia means to perceive, while proprioception is not necessarily conscious but may occur as unconscious sensory receptions which elicit reflex reactions. This conception places kinesthesia as a higher-order derivative which calls on proprioception for its data.

Much of Sherringtons (1906) research which distinguished the term proprioception focused on reflex actions produced when stimulating particular receptors. McCloskey (1978, p. 764) also describes that Sherrington used proprioception to refer to vestibular sensations and inputs from muscles and joints that are not necessarily perceived and other authors explicitly refer to the conscious/unconscious distinction between kinesthesia and proprioception (Ellison, 1993, p. 75; Paillard and Brouchon, 1974, p. 275). Correspondingly, in Lee and Lishmans studies of vision and body movement, they use:

visual kinaesthesia (Lishman and Lee, 1973) when they are studying subjects conscious perceptions of their own self-motion, whereas they use
visual proprioception (Lee and Lishman, 1975) when they are studying subjects unconscious, reflexive responses for maintaining upright posture.

Research has also focused on whether sensory discharges from muscle spindle receptors have any direct access to conscious perception (kinesthesia) or are used solely for subconscious reflexive control of movement (proprioception). This question has been referred to as the problem of conscious proprioception, whether there is awareness of muscle length and tension changes (Gelfan and Carter, 1967).

Some evidence indicates that sensory reception from muscles is not consciously perceived. Anaesthestized joints produces a loss of perception of passive movement or position in the finger joint (Provins, 1958) or the toe (Browne et al., 1954) even though the muscles which act upon these joints were unaffected by the anaesthesia. Stretching a muscle by pulling on the exposed tendon does not produce any conscious perception of limb movement in the fingers, hand, or foot and so Gelfan and Carter (1967) conclude that there is no muscle sense in man. This effect was duplicated by Moberg (1983) who stresses the importance of skin receptors (rather than joint or muscle receptors) for conscious kinesthesia in the fingers and hand.

However other evidence indicates that muscles do play a role in conscious perception. Sensory impulses from muscle spindle receptors have been found to have direct connections to the cerebral cortex in baboons (Phillips et al., 1971) and cats (Oscarsson and Rosen, 1963). When muscles acting on the fingers are lightly tensed or voluntarily moved then motion is perceived even if the joints and skin have been paralysed (Goodwin et al., 1972a; 1972b). This sensory facilitation of actively moved versus passively manipulated muscles was also noted earlier (Browne et al., 1954). Illusions of forearm movements and false positions have also been elicited by vibrating the muscles and tendons with a physiotherapy vibrator (Goodwin et al., 1972a; 1972c).

Distinguishing receptors as to whether their stimulation become conscious or unconscious appears to be a tentative affair. This is especially true since conscious perceptions rarely arise solely from the sensations of one individual receptor, especially in kinesthesia where input from an abundance of receptors is combined into a unified perception. Conscious kinesthesia is not attributed to particular receptors per se, but as a phenomenological experience of the bodys positions, motions, forces etc. In McCloskeys (1978) exhaustive review of kinesthetic sensibility, and in particular the question of Are muscles sentient?, it is noted that perceptions are not experienced in the receptors, but in the objects perceived:

. . . we are no more likely to feel kinesthetic sensations in our muscles or joints than we are to hear sounds in our heads or see objects in our retinas - but

[conscious kinesthesia] would be sensations of movement, or force, or tension, or of altered position in the parts moved by the muscles. (McCloskey, 1978, p. 777 [emphasis his])

It would not be beneficial for the raw data from receptors to be available to consciousness since the data from collections of receptors must be interpreted relative to each other and to exterior forces (gravity, momentum, external objects) and relative to any motor commands which have been executed. These will all influence the significance of any isolated receptor response:

. . . the essential point is that it would be of little value for the highest sensory centres to receive raw data from the muscle afferents, because what these mean depends entirely upon what the relevant muscle is being told to do by the motor system.

(Goodwin et al., 1972a, p. 744)

Likewise, in a study of the history of proprioception Dickinson (1974, p. 10) concludes that at a physiological level, the absence of a direct link from receptors to the cortex may not necessarily preclude some indirect participation in perception since the perception is derived at an unconscious level anyway.

Conclusions; Working Definitions.

From the overlapping concepts of somaesthesia, kinesthesia, proprioception, etc. outlined above, the following working definitions can be used in an integrated model.

Proprioception is not favored since it belongs to an interior/exterior distinction which has been shown to be invalid. In Dickinsons (1974) historical review of proprioception it is observed that Not only is there disagreement concerning the definition of proprioception, there is even disagreement over whether proprioception may be viewed as a sensory modality (p. 9). Clark and Horch (1986, p. 13.2) state that the term proprioceptive lacks a precise definition and therefore they prefer kinesthesia.

Kinesthesia will be used in its broadest sense to refer to perceptions arising from muscle, tendon, joint, skin, vestibular, visual, and audio receptors. In addition, an interior knowledge of motor commands or efferent data can be considered to be another source of kinesthetic information (see below).

Other senses can be conceived to be kinesthetic sub-systems. These include limb position sense, limb movement sense, sense of linear or rotary self-motion, sense of balance or equilibrium, and the sense of force.

Somatic is generally used in its typical definition of referring to perceptions arising from within the physical mass of the body itself, namely from receptors in muscles, tendons, joints, and skin. These receptors comprise a complete grouping in themselves within the larger group of kinesthetic receptors. This somatic system is synonymous with the haptic system but since haptic comes from to touch it is more associated with skin receptors rather than the integrated functioning of receptors within the somatic system.